This invention relates generally to acoustic wave transmitters, and more particularly the invention relates to a low impedance acoustic source suitable for use in down-hole well logging.
Acoustic wave transmitters are used in various applications such as underwater wave transmitters in sonar applications, for example, and in down-hole acoustic well logging. The advent of new piezoelectric and structural materials and solid state electronic devices has lead to substantial improvements in down-hole acoustic transducer performance with regard to frequency range, sensitivity, depth of submersion, and power output. Theory has kept abreast of these developments, particularly in revealing the significance of hole wave forms. This is especially true in the single well acoustic well logging field.
A critical problem especially in cross well logging is impedance matching of the acoustic source to the media. Acoustic impedance is defined as: ##EQU1## Where; P: Pressure;
u: particle velocity; PA1 p: density; PA1 V: acoustic velocity.
Common piezoelectric ceramics, like PZT-4, has the acoustic impedance V.sub.1 .times..rho.=4.6.times.10.sup.3 m/s.times. 7.5.times. 10.sup.3 kg/m.sup.3 =34.5.times.10.sup.6 kg/s.multidot. m.sup.2, which is about 20 times larger than acoustic impedance of bore hole mud; therefore, the coupling of acoustic energy is not well matched.
One way to lower the impedance is to change u and P, since the density .rho. and the acoustic velocity V have been set in the way of manufacturing. A kind of piezoelectric crystal has its fixed acoustic impedance, while u and P could be changed by means of a suitable mechanical transformer.
Particle velocity can be defined in terms of displacement, .DELTA.x, and crystal frequency, f, as follows: ##EQU2##
The piezoelectric crystal frequency f is also fixed for a specific crystal, and too high frequency will cause the phenomenon of cavitation especially in the water medium at the vibrating surface of the transducer.
Accordingly, an object of the present invention is an improved acoustic source.
Another object of the invention is a low impedance down-hole acoustic source for well logging.
A feature of the invention is a mechanical transformer which increases particle velocity.
Another feature of the invention is a symmetrical compliant metal mechanical transformer for matching impedances.
Yet another feature of the invention is a free-flooded cavity structure for deep well submergence.